The present invention relates to near field RF communicators and NFC communicators comprising demodulation circuitry having digital signal mixers and to devices comprising such communicators and to digital signal mixing for demodulation, in particular to digital signal mixing for demodulation.
Near field RF (radio frequency) communication is becoming more and more commonplace as is the use of such technology to transfer data. Near field RF communicators communicate through the modulation of the magnetic field (H field) generated by a radio frequency antenna. Near field RF communication thus requires an antenna of one near field RF communicator to be present within the alternating magnetic field (H field) generated by the antenna of another near field RF communicator by transmission of an RF signal (for example a 13.56 Mega Hertz signal) to enable the magnetic field (H field) of the RF signal to be inductively coupled between the communicators. The RF signal may be modulated to enable communication of control and/or other data. Ranges of up to several centimetres (generally a maximum of 1 metre) are common for near field RF communicators.
NFC communicators are a type of near field RF communicator that is capable in an initiator mode of initiating a near field RF communication (through transmission or generation of an alternating magnetic field) with another near field RF communicator and is capable in a target mode of responding to initiation of a near field RF communication by another near field RF communicator. The term “near field RF communicator” includes not only NFC communicators but also initiator near field RF communicators such as RFID transceivers or readers that are capable of initiating a near field RF communication but not responding to initiation of a near field RF communication by another near field RF communicator and target or responding near field RF communicators such as RFID transponders or tags that are capable of responding to initiation of a near field RF communication by another near field RF communicator but not of initiating a near field RF communication with another near field RF communicator. Hence NFC communicators can act as both RFID transceivers and RFID transponders and are able to communicate with other NFC communicators, RFID transceivers and RFID transponders.
In addition NFC communicators may be associated with or comprised within or attached to certain peripheral devices, for example SIM cards (e.g. UICC), Secure Elements, memory devices (for example MCU, RAM, ROM and non-volatile memory), display driver or other drivers. During operation the NFC communicator must also be able to communicate with and transfer data to and from such peripheral device.
There are several standards in existence which set out certain communication protocols and functional requirements for RFID and near field RF communications. Examples are ISO/IEC 14443, ISO 15693, ISO/IEC 18092 and ISO/IEC 21481.
NFC communicators may be comprised within a larger device, NFC communications enabled devices. Examples include mobile telephones, PDAs, computers, smart cards. When comprised within such NFC communications enabled devices the NFC communicator must be able to transfer data to and from the larger device and to and from any peripheral devices (including interface systems, such as the single wire protocol) associated with such larger device.
A modulated signal comprises a data signal modulated on to a carrier signal. Data may be modulated on to a carrier signal according to a pulse position modulation scheme and/or amplitude modulation scheme. In the case of digital signal encoding, such schemes are sometimes referred to as phase shift keying and amplitude shift keying respectively. Some modulation schemes, such as quadrature amplitude modulation, are equivalent in effect to a mixture of amplitude and phase modulation. To extract a data signal from a modulated signal (that is to demodulate the signal) it has been proposed to mix the modulated signal with two mixing signals having a phase delay of 90° between the two signals. This is referred to as in-phase, quadrature-phase mixing, or IQ-mixing. Phase information about the modulated signal can then be derived based on the relative signs and magnitudes of the in-phase and quadrature-phase mixed signals. In other examples, other (smaller) phase shift angles may be used to provide higher order encoding.
Where the phase angle between mixing signals is not accurately known (for example where the phase angle between the I and Q mixing signals is not 90°) the demodulation may be compromised.
In digital signal processing it has been proposed to derive mixing signals by down-sampling a high frequency clock signal. To provide a phase delay of exactly a quarter of a cycle (90°) between mixing signals at the carrier frequency the frequency of the clock signal should be an exact multiple of four times the carrier frequency, i.e. it should be possible to provide a quarter cycle phase shift in an integer number of clock cycles. For example, if the frequency of the clock signal is 24 times higher than the required mixing signals, it is possible to obtain two such mixing signals each offset from the other by one quarter of a cycle of the carrier (6 cycles of the clock signal) by down-sampling the clock signal by a factor of 24. An example of this process is shown in FIG. 5.
The inventors in the present case have appreciated that, where a clock signal is not a multiple of 4 times greater than the required mixing frequency, then it is not possible to obtain the required quarter cycle phase-offset in the mixing signals from an integer number of cycles of the clock signal.
One manifestation of this problem is that the phase shift between the in-phase (I) and quadrature phase (Q) mixing signals is not a perfect quadrature (90° or π/2) shift and/or the duty cycle of the signals may be spoilt, for example the time integral of one or both of the mixing signals (or the product of the two) may be non zero over a whole number of cycles. In other words, the orthogonality condition upon which accurate IQ demodulation depends is not met because of discretization errors.